1. Field of the Invention
This invention relates to an apparatus for driving a liquid crystal display (LCD) panel, and more particularly, to a voltage generating circuit for generating a plurality of voltage signals required to drive the LCD panel.
2. Description of the Conventional Art
A conventional LCD panel controls transmission of a light beam from a light source according to an input video signal to display a picture corresponding to the input video signal. The conventional LCD panel includes liquid crystal cells arranged in a matrix pattern and control switches for selectively activating the cells to receive the input video signals.
A driving apparatus is provided in the conventional LCD to actuate the control switches for activating the liquid crystal cells. The driving apparatus changes the polarity of video, voltage signals applied to the cells between a positive(+) and negative(-) polarity according to a set voltage level. This reduces the amount of driving voltage needed to drive the LCD panel and avoids degradation of liquid crystal. To change the polarity of video voltage signals, the driving apparatus must supply voltage signals for controlling the control switches and also a common voltage having a constant voltage level to each liquid crystal cell.
To generate the common voltage and the voltage signals for the control switches, a conventional LCD driving apparatus requires many different voltage generating circuits.
As shown in FIG. 1, the conventional LCD panel includes a plurality of thin film transistors (TFTs) 10 arranged at crossovers where gate lines 11 intersect data lines 13, a plurality of liquid crystal cells 12 each connected between the source of a correspondence TFT 10 and the common voltage Vcom, a plurality of support capacitors 14 each connected in parallel with the corresponding liquid crystal cell 12, a plurality of gate drivers 16 connected to the gate lines 11, and a plurality of data line drivers 25 for supplying video signals to the data lines 13. The LCD panel further includes a first pad 15 for inputting the common voltage Vcom, second pads 17 for inputting a gate floating voltage Vst, a third pad 19 for inputting a first gate driving voltage Vgh, and a fourth pad 21 for inputting a second gate driving voltage Vgl.
As shown in FIG. 2, each of the gate drivers 16 includes an NMOS transistor 18 and a PMOS transistor 20 for commonly receiving a gate control signal from a gate control line 23. The NMOS transistor 18 transfers the first gate driving voltage Vgh from the third pad 19 to the gate line 11 when the gate control signal has a logical value of "1". On the other hand, the PMOS transistor 20 transfers the second gate driving voltage Vgl from the fourth pad 21 to the gate line 11 when the gate control signal has a logical value of "0".
To generate the common voltage Vcom, the gate floating voltage Vst and the first and second gate driving voltages Vgh and Vgl required by the LCD panel, the conventional LCD panel requires separate voltage generating circuits as shown in FIGS. 3 to 5. These voltage generating circuits included in the conventional LCD panel will be explained below referring to FIGS. 3 to 5.
First, the common voltage Vcom is commonly supplied, via the first pad 15, to a number of liquid crystal cells 12 and support capacitors 14. The gate floating voltage Vst is commonly supplied, via the second pads 17, to the gate lines 11. The common voltage Vcom and the gate floating voltage Vst are produced by a first voltage generating circuit as shown in FIG. 3.
The first voltage generating circuit includes an operational amplifier A1 for differentially amplifying a reference signal Vref and a horizontal synchronous signal Hsy, push-pull amplifiers Q1 and Q2 for further amplifying an output signal of the operational amplifier A1, and a resistor R1 and capacitor Q1 connected in parallel with each other for feeding back the output signal of the push-pull amplifiers Q1 and Q2 to be added to a line pulse LS. These push-pull amplifiers Q1 and Q2 generate the output signal of the operational amplifier A1 by utilizing a high level supply voltage +Vcc and a low level supply voltage -Vcc. Each output signal of the push-pull amplifiers Q1 and Q2 is applied to the first pad 15 (FIG. 1) as a common voltage Vcom or to the second pad 17 as a gate floating voltage Vst. The voltage level of each output signal of the push-pull amplifiers Q1 and Q2 is determined by the voltage level of the reference voltage Vref.
Second, the first gate driving voltage Vgh is commonly supplied, via the third pad 19, to the gate drivers 16 and is generated by a second voltage generating (or clamping) circuit as shown in FIG. 4. The second voltage generating circuit includes a diode D1 connected between the high level supply voltage source Vcc and the third pad 19, and a capacitor C2 connected between a line pulse (LS) input node HIN and the third pad 19. The capacitor C2 accumulates a difference between the voltage of line pulse LS and the high level supply voltage supplied through the diode D1 from the high level voltage source Vcc. As a result, the first gate driving voltage Vgh changing in accordance with a logical value of the line pulse LS is generated and supplied to the third pad 19.
Finally, the second gate driving voltage Vgl is commonly supplied, via the fourth pad 21, to the gate drivers 16 and is generated by a third voltage generating (clamping) circuit as shown in FIG. 5. The third voltage generating circuit includes a diode D2 connected between a low level supply voltage source -Vcc and the fourth pad 21, and a capacitor C3 connected between the line pulse (LS) input node HIN and the fourth pad 21. The capacitor C3 accumulates a difference between the voltage of line pulse LS and the low level supply voltage applied through the diode D2 from the low level voltage source -Vcc. The second gate driving voltage Vgl changing in accordance with a logical value of the line pulse LS is generated and supplied to the fourth pad 21.
As described above, the conventional LCD panel driving apparatus requires at least several voltage generating circuits to generate all of the control voltage signals required to drive the LCD panel. This results in a complicated circuit configuration and more frequent circuit failures.